Correct unspecified enum constant values

[tejaskochar-db]

Summary

Corrects generated unspecified enum constants so their runtime value is the empty string instead of the proto symbolic name. This is a breaking correction to the beta API surface.

Why

Unspecified enum members represent absence. Exposing their symbolic names as runtime values made them look like sendable wire values, even though callers should omit the field instead. Using "" aligns the constants with absence and prevents the SDK from presenting a sentinel as an explicit value to send.

What changed

Interface changes

• Breaking (beta correction): Generated *_UNSPECIFIED constants now equal "" instead of their symbolic names.

Behavioral changes

• Serializing an unspecified enum constant now produces the empty value that represents absence.

Internal changes

• Regenerated the affected API model files.
• Added the same API change notice to each affected package changelog.

How is this tested?

npm run ci passes, including builds, linting, formatting checks, type checking, Node tests, browser tests, and license validation.